High-specific-gravity EPDM composition, dynamic damper made from the composition, tennis racket with the dynamic damper, and radiation-shielding material comprising the composition

ABSTRACT

A high specific-gravity EPDM composition consisting of a mixture of EPDM containing diene at less than 4.5 wt % and ethylene at not less than 58 wt % nor more than 80 wt % and having a Mooney viscosity ML 1+4  not less than 50 nor more than 170 at 125° C. and a powdery material, containing powder whose specific gravity is not less than 12 as a main component thereof, added to the EPDM at not less than 80 wt % nor more than 97.5 wt % of a whole amount of the high specific-gravity EPDM composition.

This application is the national phase under 35 U.S.C. §371 of PCTInternational Application No. PCT/JP02/06044 which has an Internationalfiling date of Jun. 17, 2002, which designated the United States ofAmerica.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a high specific-gravity EPDMcomposition, a dynamic damper composed of the high specific-gravity EPDMcomposition, a dynamic damper-installed tennis racket, and radioactiveray shielding material composed of the high specific-gravity EPDMcomposition. More particularly, the high specific-gravity EPDMcomposition of present invention has improved moldability andweatherability to be used as the dynamic damper that is mounted onsports goods and the like and as the radioactive ray shielding materialof radiographic inspection appliances and the like.

2. Description of the Related Art

The dynamic damper (vibration-damping material) is often used to reduceand relieve impacts and vibrations generated in sporting ball-hittinggoods and the like when they are used. For example, in the tennisracket, the dynamic damper is fixed to the racket frame thereof toresonate the dynamic damper with vibrations of the racket frame when thetennis racket hits a tennis ball. Thereby the vibrations generated by animpact are relieved to reduce vibrations to be transmitted to a player'shand. In this manner, occurrence of tennis elbow is suppressed.

For example, the present applicant proposed a dynamic damper to beinstalled on the tennis racket as disclosed in Japanese PatentApplication Laid-Open No. 2002-085598 and No. 2002-048185. In theproposal, as the material for the mass-adding part of the dynamicdamper, the present applicant proposed a thermoplastic elastomer andchloroprene rubber in which lead and tungsten are dispersed. However,the lead which has been hitherto used in a large amount is inexpensivebut the use mode thereof and the use amount thereof are restricted toprevent environmental pollution.

There is a fear that a tennis player touches the dynamic damper orstrikes the dynamic damper against others by mistake. In the case wherethe dynamic damper is formed of a hard material such as metal, there isa possibility that the tennis player is injured, which is notpreferable. Ordinary metals have a comparatively low specific gravity.Thus when they are used as the mass-adding part of the dynamic damper,the dynamic damper has a large volume, which disturbs the player duringthe use of the tennis racket. For appearance, it is preferable that thedynamic damper is small.

Therefore the tennis player desires a material not pollutingenvironment, soft, having a high specific gravity, and having acomparatively high strength so that it is not broken when it drops orstrikes against an object. For example, if the mass-adding part of thedynamic damper is sheet-shaped and its thickness is as thin as 0.6 mm toreduce its volume, it is necessary for the material having a highspecific gravity to be soft in such an extent that the material flowssmoothly in a molding die and have a strength in such an extent that itis not broken, even though it is thin and sheet-shaped. Because thedynamic damper is formed by combining the mass-adding part and theviscoelastic part with each other, it is necessary that the material hasadhesion to other materials.

For applications other than the dynamic damper, there is a demand forthe development of a material which has a high specific gravity (4-13)and is soft. Thus in recent years, a large number of rubbers andresinous materials having a high specific gravity are proposed.

For example, in Japanese Patent Application Laid-Open No.2000-27331,there is proposed a vibration-damping/sound insulation sheet to whichslurry containing a large amount of a high specific gravity filler andan emulsion of rubber is applied

In the field of shielding radioactive rays, in addition to a radioactiveray-shielding protection cloths used for medical purposes, only apredetermined portion is irradiated with a necessary amount of theradioactive rays in radiation therapy and measurement and portion whichis not necessary is not irradiated. This is because there is a demandfor prevention of destruction of normal cells and prevention ofexcessive exposure to radiation. As such, a radioactive ray shieldingmaterial is used in portions other than the portion to be irradiatedwith the radioactive rays. In fields other than the medical field, theradioactive ray shielding material is also used to shield theradioactive ray in inspections of food, inspections at a custom house,and techniques of analyzing objects without destroying them.

Heretofore, a material in which lead, a lead compound, a lead alloy orantimony is blended in resin or rubber is generally used for theradioactive ray shielding protection cloths. An acrylic plate or thelike has been used for comparatively weak radioactive rays. A tungstenand a plate made of an alloy thereof are also used as the radioactiveray shielding material.

As disclosed in Japanese Patent Application Laid-Open No.8-122492, thereis proposed a radioactive ray shielding material made of a resincontaining a plasticizer in which tungsten is dispersed.

As disclosed in Japanese Patent Application Laid-Open No.10-153687,there are proposed vulcanized fluororubber and EPDM rubber bothcontaining tungsten dispersed therein and also chloroprene rubbercontaining tungsten dispersed therein.

However, the method of applying the emulsion to thevibration-damping/sound insulation sheet disclosed in Japanese PatentApplication Laid-Open No.2000-27331 is capable of forming a coatingfilm, but is incapable of forming molded products thick or complicatedin configuration.

In the radioactive ray shielding material disclosed in Japanese PatentApplication Laid-Open No.8-122492, tungsten may settle in the process ofdrying and removing a solvent. Thus there is a room for improvement inheat resistance and strength of the radioactive ray shielding material.

As described above, as the material for the radioactive ray shieldingmaterial, the vibration-damping/sound insulation sheet, the soundproofmaterial, and the like, there is a demand for development of a materialwhich is soft and has a high specific gravity. However, the proposedmaterials are all hard and difficult to handle and moreover there is aroom for improvement in unpollutability, moldability, andprocessability.

In the radioactive ray shielding field, in the case where lead or itsalloy is used, it is necessary to prepare a casting mold to process thelead or its alloy into a predetermined configuration and dissolve thelead or its alloy in the casting mold to cast it. A lead-casting work isvery costly because it is necessary to dissolve the lead or its alloyand manufacture the casting mold. Further the dissolving the lead or itsalloy causes environment around a work place to deteriorate and affect ahuman body adversely.

In the case where the radioactive ray shielding material is used forinspection of food or the like, when the radioactive ray shieldingmaterial is used in contact with a human body directly or indirectly,there is a fear that the lead which has separated from the radioactiveray shielding material contaminates environment. The melting point ofthe lead alloy is as low as 80° C. Thus in the case where theradioactive ray shielding material having the lead alloy is used for amedical purpose, a medical appliance or the like having the radioactiveray shielding material cannot be heated at about 100° C. although it isnecessary to sterilize it in hot water. Further the radioactive rayshielding material having the lead alloy cannot be used for a pipe orthe like of an atomic power plant or the like, because the pipe isrequired to have heat resistance at 200° C. As described above, in theradioactive ray shielding field, there is a demand for the developmentof a material which is high in specific gravity, the performance ofshielding radioactive rays, strength, moldability, and workability.

Chloroprene in which tungsten is dispersed is a little bad inweatherability. Thus in an outdoor exposure test (sunshine) which isconducted in a strict condition and in a sunshine weatherometer test ofexposing an object to ultraviolet rays, tungsten and additives areliable to bloom or whiten.

The vulcanized fluororubber disclosed in Japanese Patent ApplicationLaid-Open No.10-153687 is elastic, soft, strong to some extent, andsuperior in weatherability, but has a room for improvement in adhesionperformance and in a process of combining other materials and thefluororubber with each other. The EPDM rubber also disclosed in JapanesePatent Application Laid-Open No. 10-153687 has rubber elasticity and ahigh strength, is superior in weatherability, and more adhesive than thefluororubber. Thus the EPDM rubber can be processed in combination withan adhesive agent or other materials, but has a room for improvement inmoldability, processability, and weatherability in the case where metalpowder having a high specific gravity is dispersed in the EPDM rubber.

SUMMARY OF THE INVENTION

The present invention has been made in view of the above-describedproblems. Thus it is a first object of the present invention to providea high specific-gravity EPDM composition which is soft and high inspecific gravity, moldability, processability, weatherability, andstrength.

It is a second object of the present invention to provide a dynamicdamper that has a small volume, is high in vibration-dampingperformance, and is preferably attached to sports goods.

It is a third object of the present invention to provide a tennis racketon which a dynamic damper small and thin, not disturbing a player inplaying tennis, and superior in operability is installed.

It is a fourth object of the present invention to provide a radioactiveray shielding material which does not pollute environment and is high inmoldability and processability, strength, and radioactive ray shieldingperformance.

To achieve the first object, the present invention provides a highspecific-gravity EPDM composition consisting of a mixture of EPDMcontaining diene at less than 4.5 wt % and ethylene at not less than 58wt % nor more than 80 wt % and having a Mooney viscosity ML₁₊₄ not less50 nor more than 170 at 125° C. and a powdery material,

containing powder whose specific gravity is not less than 12 as a maincomponent thereof, added to the EPDM at not less than 80 wt % nor morethan 97.5 wt % of a whole amount of the high specific-gravity EPDMcomposition.

The molecular weight, the amount of the diene, and the amount of theethylene differentiate the characteristic of the EPDM. The invention isbased on the present inventors' finding, made as result of experiments,that it is possible to obtain the high specific-gravity EPDM compositionthat is soft and high in specific gravity, moldability, processability,and weatherability, and strength by using the EPDM which is specified tothe above-described range in its diene amount, ethylene amount, andMooney viscosity and by adding the powdery material containing powderwhose specific gravity is not less than 12 as its main component to theEPDM at a wt % in the above-described range.

The amount of the diene is less than 4.5 wt % (wt % of diene componentof entire material of EPDM) and preferably less than 3.5 wt %.

In an outdoor exposure test (sunshine) and a sunshine weatherometer testof exposing the high specific-gravity EPDM composition to ultravioletrays, the powdery material such as tungsten and the like and an additivecontained in the EPDM blooms in the case where the EPDM containing muchdiene is used. However, by reducing the diene amount of the EPDM to lessthan 4.5 wt %, it is possible to suppress generation of the blooming andimprove the weatherability of the high specific-gravity EPDMcomposition.

To form the high specific-gravity EPDM rubber at a required strength, itis important to optimize the range of the amount of the ethylene and therange of the Mooney viscosity. It is preferable to use the EPDMcontaining a large amount of the ethylene and having a high Mooneyviscosity.

Therefore the weight percentage of the ethylene of the entire materialof the EPDM is set to not less than 58 wt % nor more than 80 wt % andfavorably not less than 64 wt % nor more than 80 wt % and more favorablynot less than 64 wt % nor more than 70 wt %. Most favorable range of theweight percentage of the ethylene is not less than 64 wt % nor more than66 wt %.

If the EPDM contains less than 58 wt % of the ethylene, the EPDM has alow strength. Thus in molding the high specific-gravity EPDM compositioninto a sheet after the powdery material is dispersed in the EPDM, themoldability of the high specific-gravity EPDM composition is low. On theother hand, if the EPDM contains more than 80 wt % of the ethylene, theEPDM is hard. Therefore it is difficult to disperse the powdery materialsuch as tungsten and the like uniformly in the EPDM. In this case, inmolding the high specific-gravity EPDM composition into a thin sheet,its moldability is liable to deteriorate and a product such as a dynamicdamper formed by molding the high specific-gravity EPDM composition ishard. Thus the product strikes against a human body strongly.

The Mooney viscosity ML₁₊₄ of the EPDM at 125° C. is set to not lessthan 50 nor more than 170 and favorably to not less than 100 nor morethan 170 and more favorably to not less than 150 nor more than 165.

The Mooney viscosity is measured by the method provided by JIS K6300 andused as an index indicating a viscosity. M of ML₁₊₄ is the first letterof Mooney and L of ML₁₊₄ is the first letter of L-type rotor. (1+4) ofML₁₊₄ means a preheating time period of one minute and a rotation timeperiod (four minutes) of a rotor.

If the Mooney viscosity is less than 50 at 125° C., the EPDM has a lowstrength. Thus the mixture of the EPDM and the powdery material such asthe tungsten and the like dispersed therein has a low moldability.

On the other hand, the EPDM having the Mooney viscosity more than 170 at125° C. is hard. Thus it is difficult to accomplish uniform dispersionof the powdery material in the EPDM. Consequently the highspecific-gravity EPDM composition has a low moldability in molding itinto a thin sheet. Thus the product formed by molding the highspecific-gravity EPDM composition is hard and strikes against a humanbody strongly.

The molecular weight of the EPDM can be determined to some extent, basedon the Mooney viscosity. The more the Mooney viscosity, the more themolecular weight of the EPDM. When the Mooney viscosity is 50, themolecular weight of the EPDM is 300,000-400,000. When the Mooneyviscosity is 170, the molecular weight of the EPDM is about 600,000.

The EPDM is mixed with the powdery material, containing the powderhaving the specific gravity not less than 12 as its main component, atnot less than 85 wt % of the weight of the entire high specific-gravityEPDM composition (total weight of EPDM, additives, and powdery material)nor more than 97.5 wt %.

If the mixing amount of the powdery material containing the powderhaving the specific gravity not less than 12 as its main component isless than 85 wt % of the entire weight of the high specific-gravity EPDMcomposition, the specific gravity of the entire high specific-gravityEPDM composition is not so high. In this case, in adding a necessarymass to the mass-adding part of the dynamic damper, it is necessary tomake the mass-adding part large. Consequently the volume of the dynamicdamper becomes large.

On the other hand, if the mixing amount of the powdery materialcontaining the powder having the specific gravity not less than 12 ismore than 97.5 wt % of the entire weight of the high specific-gravityEPDM composition, the EPDM is incapable of covering the surface of thepowdery material. Consequently the strength of the high specific-gravityEPDM composition and its moldability are low.

In the case where the high specific-gravity EPDM composition containsthe powdery material containing the powder having a specific gravity notless than 12 as its main component, it is possible to increase thespecific gravity of the high specific-gravity EPDM compositionefficiently. Thus it is possible to reduce the volume of a productformed by molding the high specific-gravity EPDM composition. From theabove viewpoint, the powder having the specific gravity not less than 12is contained in the powdery material at not less than 70 wt % andpreferably at not less than 80 wt % of the weight of the entire powderymaterial.

To disperse the powdery material in the EPDM favorably to increase thestrength of the high specific-gravity EPDM composition, the averageparticle diameter of the powder is favorably less than 50μm and morefavorably less than 20μm. The flowability and moldability of the highspecific-gravity EPDM composition can be increased by using powderhaving a small diameter, for example, less than 5 μm in combination withpowder having a large diameter, for example, more than 27μm.

Tungsten, a tungsten compound or a tungsten based alloy is preferable asthe powdery material containing the powder having the specific gravitynot less than 12.

Of metal materials, tungsten has a high specific gravity, is unharmfulto a human body, inexpensive, and easily obtainable. Thus the tungstencan be preferably used. Since the tungsten has a high specific gravity,it is preferable that the powdery material consists of thetungsten(100%). However it is possible to use the tungsten compound orthe tungsten based alloy. Alternatively a mixture of the tungsten, thetungsten compound, and the tungsten based alloy may be used. Thespecific gravity of the tungsten is 19.3.

The powdery material such as the tungsten or the like not chemicallysurface-treated by a coupling agent can be preferably used. For example,the powdery material can be preferably used in a physically treatedstate or in a powdered state.

If the powdery material is chemically surface-treated with the couplingagent, the powdery material may bloom or the strength of the highspecific-gravity EPDM composition may deteriorate.

To make the high specific-gravity EPDM composition soft, it ispreferable to add an appropriate amount of oil and the like to the EPDMas a softener.

In adding the softener to the EPDM, less than 150 wt % of the softeneris added to 100 wt % of the EPDM.

The softener does not necessarily have to be added to the EPDM. But ifnot less than 150 wt % of the softener is added to 100 wt % of the EPDM,blooming is liable to occur in a weatherability test.

The specific gravity of the high specific-gravity EPDM composition ofthe present invention is set to not less than 4.5 nor more than 13.1 andpreferably not less than 5.0 no more than 9.5.

If the specific gravity of the high specific-gravity EPDM composition isless than 4.5, i.e., if the specific gravity thereof is low, the volumeof the dynamic damper or he like is large. Thus the dynamic damper orthe like disturbs a player in using a tennis racket. On the other hand,if the specific gravity of the high specific-gravity EPDM composition ismore than 13.1, it is necessary for the high specific-gravity EPDMcomposition to contain much powdery material, which makes it difficultto process the high specific-gravity EPDM composition.

It is preferable that the surface hardness of the vulcanized highspecific-gravity EPDM composition measured by the method specified byJIS K-6253 (tester durometer type A) is less than 90.

If the hardness of the high specific-gravity EPDM composition is morethan 90, it is so hard that it is difficult to mold the highspecific-gravity EPDM composition integrally with a material for theviscoelastic part constituting the dynamic damper. Supposing that thehigh specific-gravity EPDM composition satisfies other demandedcharacteristics, the lower the surface hardness, the better.

It is preferable that the tensile strength of the high specific-gravityEPDM composition is not less than 3 MPa. If the tensile strength thereofis less than 3 MPa, the high specific-gravity EPDM composition is liableto crack or break in molding it into the dynamic damper or the like andwhen the dynamic damper or the like composed thereof is used. Supposingthat the high specific-gravity EPDM composition satisfies other demandedcharacteristics, in the condition that it is not less than 3MPa, thehigher the tensile strength, the better.

The main chain of the EPDM consists of saturated hydrocarbon and doesnot contain a double bond. Thus even though the EPDM is exposed to anozone atmosphere having a high concentration or a light-irradiatedenvironment for a long time, the main chain is not likely to cut. Thusthe high specific-gravity EPDM composition has a high weatherability. Inthe present invention, the kind and the like of the diene component ofthe EPDM is not limited to a specific one.

From the viewpoint of weatherability, the less the amount of the diene,the better. EPM containing no diene is superior to the EPDM in thisrespect. However, since the EPM cannot be vulcanized with sulfur, it isnecessary to vulcanize the EPM with peroxide. Since the speed of thevulcanization with the peroxide is lower than that of the vulcanizationwith the sulfur, the use of the EPM vulcanized with the peroxide causesworkability to be low. Therefore the EPDM vulcanized with the sulfur ismore favorable than the EPM in consideration of weatherability andworkability.

As the EPDM, it is possible to use both non-oil-extended type consistingof a rubber component and oil-extended type containing the rubbercomponent and oil. The weight of the oil of the EPDM of the oil-extendedtype added to the EPDM is treated as the added weight (amount of oil) ofthe softener.

The oil to be used as the softener is not limited to a specific one.Paraffin oil and naphthenic oil compatible with the EPDM are preferable.In addition, it is possible to use known synthetic oil such as mineraloil of aromatic series, oligomer of hydrocarbon series or process oil.As the synthetic oil, it is possible to use oligomer of alpha olefin,oligomer of butane, and amorphous oligomer of ethylene and alpha olefin.

As the vulcanizing agent, sulfur is suitable because it has a highvulcanization speed and high workability. As an accelerator, it ispreferable to use 2-mercapto-benzothiazole, tetraethylthiuram disulfide,zinc dibutyl-dithiocarbamate, and tellurium diethyldithiocarbamate byappropriately combining them with each other. Thereby the rubbercomponent can be efficiently cross-linked.

To achieve the second object, the dynamic damper of the presentinvention is composed of a viscoelastic part and a mass-adding part. Asthe mass-adding part, the high specific-gravity EPDM composition of thepresent invention is used.

As the mass-adding part, the high specific-gravity EPDM composition isused. Thus it is possible to make the volume of the dynamic damper ofthe present invention and its thickness small. It is preferable to formboth the mass-adding part and the viscoelastic part in the shape of asheet and integrate them with each other, with both parts layered oneach other. The thickness of the mass-adding part is set to 0.3 mm-2.0mm and favorably to 0.5 mm-1.0 mm. The addition of the thickness of themass-adding part and that of the viscoelastic part is set to 3 mm-5 mmand favorably to 4 mm.

The thin sheet-shaped dynamic damper does not disturb a player duringthe use of a tennis racket and is unnoticeable in appearance and doesnot prevent from playing. Further since the dynamic damper is made of asoft material, the player is not injured thereby. Furthermore thedynamic damper is so strong that it is not broken when it strikesagainst an object. Moreover because the dynamic damper has highweatherability, it can be used under the blazing sun.

As the material for the viscoelastic part of the dynamic damper, it ispreferable to use the EPDM which is the macromolecular material used forthe mass-adding part or a macromolecular material similar to the EPDM.In the case where a rubber material similar or same to the EPDM in avulcanizing temperature and a vulcanizing time period is used as thematerial for viscoelastic part, the EPDM for the mass-adding part can bebonded thereto by vulcanization in a die. That is, such a rubbermaterial is suitable for integral molding. The material for viscoelasticpart may be a foamed material. The material for viscoelastic part andthe material for the mass-adding part may be bonded to each other withan adhesive agent.

From the above-described standpoint, the EPDM can be preferably used asthe macromolecular material for the viscoelastic part. The EPDM can beused singly or in combination of other components.

In addition, one of the following rubbers or a combination thereof canbe used for the viscoelastic part: natural rubber (NR), isoprene rubber(IR), butadiene rubber (BR), styrene-butadiene rubber (SBR), chloroprenerubber (CR), acrylonitrile-butadiene rubber (NBR), carboxylated butylrubber, butyl rubber (IIR), halogenated butyl rubber (X-IIR),ethylene-propylene rubber (EPM), ethylene-polyvinyl acetate rubber(EVA), acrylic rubber (ACM, ANM), ethylene-acrylic rubber,chlorosulfonated polyethylene (CSM), chlorinated polyethylene (CM),epichlorohydrin rubber (CO), urethane rubber, silicone rubber, andfluorinated rubber and the like. Butyl rubber (IIR) is preferablebecause of the superior vibration absorption property.

As resin for the macromolecular material for the viscoelastic part,thermoplastic resin and thermosetting resin are used. The thermoplasticresin includes polyamide resin, polyester resin, urethane resin,polycarbonate resin, ABS resin, polyvinyl chloride resin, polyacetateresin, polyethylene resin, polyvinyl acetate resin, and polyimide resin.The thermosetting resin includes epoxy resin, unsaturated polyesterresin, phenol resin, melamine resin, urea resin, diallyl phthalateresin, polyurethane resin, and polyimide resin. The thermoplastic resinis more favorable than the thermosetting resin in consideration ofmoldability and because it can be recycled. It is also possible to usethermoplastic elastomers of styrene family, olefin family, urethanefamily, and ester family and the like.

The dynamic damper has a horizontal frame and a vertical frame disposedat both sides of the horizontal frame. Thus the dynamic damper islattice-shaped. The horizontal frame and the vertical frame are integralwith each other or separately formed and bonded to each other. It ispreferable that the horizontal frame is mounted on at least one face ofa racket frame in its thickness direction and that the vertical frame ismounted on both faces of the racket frame in its widthwise direction.

It is preferable that the horizontal frame is bent in the shape of aletter “U”, that one end of a bent portion of the horizontal framedisposed at both sides thereof is integral with the vertical frame orjoined therewith, and that the bent portion of the horizontal framedisposed at both sides thereof is installed on both faces of the racketin its widthwise direction. It is also preferable that the number of thehorizontal frames is not less than two and that the horizontal framesare mounted on the racket, with the horizontal frames sandwiching gutinsertion holes therebetween.

As described above, the horizontal frame and the vertical frame arecontinuous and integral with each other and lattice-shaped. Therefore inthe dynamic damper-installed racket, the vertical frame resonates mainlywith vibrations of the racket frame in a out-of-plane direction, whereasthe horizontal frame resonates mainly with vibrations of the racketframe in an in-plane direction, thus effectively suppressing vibrationsin the out-of-plane direction and the in-plane direction. That is,because the horizontal frame and the vertical frame are disposed in theshape of a lattice, the dynamic damper has improved vibration-dampingperformance, thus reducing impacts and vibrations.

The thickness direction of the racket means the direction vertical tothe gut-stretched surface. The widthwise direction of the racket meansthe direction parallel to the gut-stretched surface.

In the case where the dynamic damper is formed monolithically in theshape of a lattice, i.e., in the case where the vertical frame and thehorizontal frame are formed integrally with each other in the shape of alattice, the entire lattice resonates with the vibration of the racketframe in the in-plane direction, thus having an effect of suppressingthe vibration in the in-plane direction.

The dynamic damper of the present invention may be formed by setting alaminate of the material for the viscoelastic part and the material forthe mass-adding part in a die and bonding them to each other byvulcanization to mold them into the dynamic damper having a desiredconfiguration. Alternatively the dynamic damper of the present inventionmay be formed by molding the high specific-gravity EPDM composition intoa flat sheet and punching the sheet with a punching blade to shape thesheet into a desired configuration and bonding the viscoelastic part andthe mass-adding part to each other with an adhesive agent.

To achieve the third object, the present invention provides a tennisracket having a dynamic damper installed on at least one portion of ahead part surrounding a ball-hitting face of a racket frame or/and atleast one portion of a throat part of the tennis racket.

A player can play tennis without caring about the dynamic damper mountedon the tennis racket, thus using the tennis racket with a highoperability. Further the player can play tennis comfortably withoutbeing annoyed by unpleasant vibrations or injury such as tennis elbowand the like.

Supposing that the top position of the ball-hitting face surrounded withthe head part of the racket frame is 12 o'clock by regarding theball-hitting face as the surface of a clock, it is particularlypreferable that the dynamic damper is installed on at least one portionof the head part in such a way that the dynamic damper is disposed in anangular range of ±15° with respect to a three o'clock position and anine o'clock position. The three o'clock position and the nine o'clockposition are maximum amplitude positions of the in-plane vibration andthat of the out-of-plane secondary vibration. Thereby the dynamic damperis capable of efficiently suppressing vibrations in the in-plane andout-of-plane directions without adversely affecting the operability ofthe tennis racket.

Because a mass is applied to a wide portion (three o'clock position andnine o'clock position) of the head part, the moment of inertia aroundthe grip becomes large. Thus when a ball collides with a portion of theball-hitting face other than its center, the dynamic damper prevents therotation of the racket and reduces the degree of burden to be applied tothe player's elbow and the like.

From the viewpoint of balance, it is preferable that the dynamic damperof the present invention is installed on the racket frame at left andright positions symmetrical with respect to the widthwise center of theracket frame. But the dynamic damper-installing position is not limitedto these positions. A plurality of the dynamic dampers may be mounted onthe racket frame at the left and right positions thereof. It ispreferable to form a concavity on the dynamic damper-installingposition.

It is preferable that the racket frame is made of a fiber reinforcedresin. It is particularly preferable that the racket frame is composedof fiber reinforced prepregs layered one upon another in the shape of ahollow pipe. The racket frame may be made of materials other than thefiber reinforced resin by various manufacturing methods, for example,metal and the like.

To achieve the fourth object, the present invention provides aradioactive ray shielding material composed of the high specific-gravityEPDM composition.

The radioactive ray shielding material of the present invention iscomposed of the high specific-gravity EPDM composition, as describedabove. Therefore the radioactive ray shielding material does not polluteenvironment and has high moldability, processability, heat resistance,and strength, and radioactive ray shielding performance. Morespecifically, the performance of the radioactive ray shielding materialcan be improved by containing the powdery material having a highspecific gravity in the EPDM. Further by specifying the property of theEPDM to be mixed with the powdery material, the high specific-gravityEPDM composition is allowed to be elastic and have high moldability,processability, and durability.

Because the radioactive ray shielding material of the present inventionis superior in processability, it can be cut easily with scissors andthe like, and an opening can be easily formed. Thus the radioactive rayshielding material can be formed into various configurations. Furtherbecause the radioactive ray shielding material is flexible and iscapable of making an elastic deformation, it can be easily inserted intoa gap which requires shielding of radioactive rays in the form of asheet or the like in conformity to the configuration of the gap.Therefore it is possible to shield radioactive rays which have leakedfrom a gap or the like. Further by utilizing the elastic deformation ofthe radioactive ray shielding material, it can be easily mounted on anirregular portion and shield the radioactive rays. Furthermore since theradioactive ray shielding material has a sufficient strength, it is notbroken when it is inserted into the gap or the like and when it isprocessed. Thus it is possible to reliably obtain desired radioactiveray shielding performance.

The higher the specific gravity of the radioactive ray shieldingmaterial is, the higher the radioactive ray shielding performancethereof is. In this respect, it is possible to increase the specificgravity thereof by mixing the powdery material and the like such astungsten (specific gravity: 19.3) with the EPDM at a higher mixing rate.Thus by setting the specific gravity of the radioactive ray shieldingmaterial larger than that of lead or a lead alloy (not less than 12),the radioactive ray-shielding effect provided by the radioactive rayshielding material is almost equal to or higher than that of the lead orthe lead alloy.

The gamma ray absorption coefficient (cm⁻¹) of tungsten is about onewhen the energy of the gamma ray is 1.5 MeV. The gamma ray absorptioncoefficient (cm⁻¹) of lead is about 0.6 when the energy of the gamma rayis 1.5 MeV. Therefore the radioactive ray shielding material composed ofthe high specific-gravity EPDM composition containing tungsten powderhaving high radioactive ray shielding performance has high radioactiveray shielding performance in correspondence to the mixing rate of thetungsten.

More specifically, the radioactive ray shielding material of the presentinvention can be used by embedding it in a required portion or windingit around the required portion as a sealing material for a pipe or awall of an atomic power plant and a panel of the atomic power plant whena repair work is performed. The radioactive ray shielding material canbe also used as a guard material surrounding the periphery of an X-rayinspection machine that is used to inspect foreign matters mixed in foodand inspect baggage at a customs house. The radioactive ray shieldingmaterial can be also used in the form of shop curtain having slitsformed on a sheet. The radioactive ray shielding material can be alsoused for a syringe, gloves, protection cloths, a material coveringradioactive substances, and the like.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view showing a dynamic damper composed of a highspecific-gravity EPDM composition of the present invention.

FIG. 2A is a front view showing the dynamic damper of the embodiment ofthe present invention.

FIG. 2B is a side view showing the dynamic damper of the embodiment ofthe present invention.

FIG. 2C is a plan view showing the dynamic damper of the embodiment ofthe present invention.

FIG. 3 is a perspective view showing a state in which the dynamic damperof the present invention has been installed on a racket frame.

FIG. 4 is a plan view showing a tennis racket in which the dynamicdamper of the present invention is installed at three and nine o'clockpositions of the racket frame.

FIG. 5 is a block diagram showing a system for measuring a frequency anda damping ratio of the tennis racket.

FIG. 6 is a graph showing the relationship between a frequency and atransmission function in an analysis to be made in system for measuringa frequency and a damping ratio of the tennis racket.

FIG. 7 is a schematic view showing a measuring position for a frequencyin an out-of-plane secondary mode of the tennis racket.

FIG. 8 is a schematic view showing a measuring position for a frequencyin an in-plane tertiary mode of the tennis racket.

FIGS. 9A and 9B are an explanatory view respectively for explaining anout-of-plane secondary mode of the tennis racket.

FIGS. 10A and 10B are an explanatory view respectively for explaining anin-plane tertiary mode of the tennis racket.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The embodiments of the present invention will be described below withreference to drawings.

A high specific-gravity EPDM composition of a first embodiment of thepresent invention contains EPDM containing at 4.0 wt % of diene and 66wt % of ethylene. The EPDM has a Mooney viscosity 165 at 125° C.

A base material is obtained by mixing 200 wt % of the EPDM, 100 wt % ofoil serving as a softener, a required wt % of powdery sulfur, avulcanizing accelerator, carbon, zinc oxide, stearic acid, and an ageresister and then kneading them by an enclosed-type kneader.

Tungsten powder whose average diameter is 9 μm and specific gravity is19.3 is added in an amount of 400 g to 25 g of the base material withoutsurface-treating the tungsten powder. Then the base material and thetungsten powder are kneaded by the enclosed-type kneader to obtain thehigh specific-gravity EPDM composition. The weight ratio of the tungstento the high specific-gravity EPDM composition is 94.1%. The specificgravity of the high specific-gravity EPDM composition is 9.2.

The X-ray absorption characteristic of the high specific-gravity EPDMcomposition at 6 MeV is about 96% of that of a lead plate having athickness equal to that of the high specific-gravity EPDM compositionand about twice as large as that of a commercially available lead sheet(specific gravity: 4) having a thickness equal to that of the highspecific-gravity EPDM composition. In this way, the highspecific-gravity EPDM composition has radioactive ray shieldingperformance almost equal to that of lead or superior to that of alead-containing sheet.

The surface hardness of the vulcanized high specific-gravity EPDMcomposition measured by the method specified by JIS K-6253 (testerdurometer type A) is 72, and the tensile strength of the vulcanized highspecific-gravity EPDM composition is 5.1 MPa.

FIGS. 1 through 3 show a dynamic damper 10 according to the presentinvention. The high specific-gravity EPDM composition is used as amass-adding part 11 of the dynamic damper 10.

As shown in FIG. 1, the dynamic damper is composed of a sheet includingthe sheet-shaped mass-adding part 11 and a sheet-shaped viscoelasticpart 12 layered thereon and integral therewith. Three horizontal frames13 formed by bending the above-described sheet in the shape of “U” insection are disposed almost parallel to one another at certainintervals. Two vertical frames 14, consisting of the above-describedsheet, parallel to each other are positioned at both ends of each of thehorizontal frames 13. Thus the dynamic damper 10 is lattice-shaped.

The high specific-gravity EPDM composition is used for the mass-addingpart 11. A rubber material containing the EPDM as its main component isused for the viscoelastic part 12.

The total of the thickness of the mass-adding part 11 and that of theviscoelastic part 12 is set to the range of 2.8 mm to 7.5 mm. In theembodiment, the total thickness of both parts 11 and 12 is 4 mm. Thethickness of the mass-adding part 11 and that of the viscoelastic part12 are 0.6 mm and 3.4 mm respectively.

As shown in FIGS. 2A, 2B, and 2C, in the dynamic damper 10, the width W1of the U-shaped horizontal frame 13 of the dynamic damper 10 is 5 nm.The interval W2 between the adjacent horizontal frames 13 is 5.5 mm. Thelength L2 of the long narrow vertical frame 14 is 26 mm. The length L1(length vertical to ball-hitting face when dynamic damper is mounted ontennis racket) of the U-shaped horizontal frame 13 is 41 mm.

Supposing that the top position of the ball-hitting face F of the racketframe 2 is 12 o'clock by regarding the ball-hitting face F surroundedwith a head part 3 as the face of a clock, as shown in FIGS. 3 and 4,the dynamic damper 10 is installed at the three and nine o'clockpositions of the racket frame 2. The dynamic damper 10 is installed onthe racket frame in such a way that the longitudinal direction of thevertical frame 14 of the dynamic damper 10 is disposed parallel to thatof the racket frame 2.

More specifically, as shown in FIG. 3, the dynamic damper 10 isinstalled on the racket frame 2, with the central portion of theU-shaped horizontal frame 13 disposed on the inner surface of the racketframe 2 in its thickness direction, the bent portion of the horizontalframe 13 disposed at both sides thereof disposed on both faces of theracket frame 2 in its widthwise direction, the long and narrow verticalframe 14 disposed on both faces of the racket frame 2 in its widthwisedirection, and the surface of the dynamic damper 10 at the side of theviscoelastic part 12 thereof in contact with the surface of the innerside (gut-stretched side) of the racket frame 2. The three horizontalframes 13 parallel with one another are installed on the racket frame 2,with the horizontal frames 13 sandwiching gut insertion holes gtherebetween.

As shown in FIG. 4, the racket frame 2 of the tennis racket 1 iscomposed of the head part 3 surrounding the ball-hitting face F, athroat part 4, a shaft part 5, and a grip part 6. These parts arecontinuously formed. A yoke 7 separate from the head part 3 iscontinuous with a throat part of the racket frame 2. Thus the head part3 and the yoke 7 surround the ball-hitting face F annularly.

The racket frame 2 is composed of a hollow pipe made of fiber reinforcedresin. More specifically, the pipe is made of a laminate of fiberreinforced prepregs in which carbon fibers are impregnated with epoxyresin serving as the matrix resin.

In the embodiment, the entire length of the tennis racket 1 is set to699 mm, as shown in FIG. 4. The thickness of the head part 3 surroundingthe ball-hitting face F is set to 24 mm. The thickness of the throatpart 4 is set to 21 mm. The width of the head part 3 is set to 12 mm.The width of the throat part 4 is set to 14 mm. The thickness and widthof the portion of the racket frame 2 on which the dynamic damper 10 isinstalled are set to 21 mm and 12 mm respectively. The thickness andwidth of the portion of the racket frame 2 at both sides of each dynamicdamper-installing portion are set to 24 mm and 14.5 mm respectivelywhich are a little thicker than the thickness and width of the dynamicdamper-installing portion respectively.

As described above, since the dynamic damper that is mounted on thetennis racket is composed of the high specific-gravity EPDM compositionused as the mass-adding part, it is possible to make the volume andthickness of the dynamic damper small. Therefore a player can playtennis without caring about the presence of the dynamic damper. Furthera small air resistance acts on the dynamic damper, which allows theplayer to have high operability in using a tennis racket. Although thedynamic damper is smaller than the conventional one, the former providessufficient vibration-damping performance.

The dynamic damper of the present invention is manufactured in thefollowing process:

Initially, the high specific-gravity EPDM composition is sufficientlykneaded. Thereafter, it is heated under a pressure to shape it into asheet. Thereafter, the sheet is cut to a necessary size to obtain amixture piece for the mass-adding part of the dynamic damper. Then theobtained mixture piece is set in a die having a desired configuration.Then a material for the viscoelastic part is filled into the die. Thenthe mixture for the mass-adding part and the material for theviscoelastic part are pressed and heated. As a result, both are bondedto each other by vulcanization to obtain a sheet-shaped dynamic dampercomposed of the mass-adding part and the viscoelastic part disposedthereon.

Instead of the above-described method, it is possible to set thematerial kneaded by a mill and set it into a cavity of a die for themass-adding part. The material is shaped by press molding at a certaintemperature to obtain the material for the mass-adding part. Then thematerial for the mass-adding part is set in the die for the dynamicdamper.

In the embodiment, the dynamic damper is mounted at the three o'clockposition and the nine o'clock position of the head part of the racketframe. However, the dynamic damper may be installed on at least oneportion of the head part, of the racket frame, surrounding theball-hitting face or/and at least one portion of the throat part of theracket frame.

The composition of the racket frame is fiber-reinforced resin or metalor the like, but it is not limited to them. The dynamic damper may beapplied to all the kind of tennis racket.

In this embodiment, since the number of the horizontal frames is three,it has the shape of a Japanese character “

”. The number of the horizontal frames may be two. In this case, thedynamic damper is rectangular. In the case where the dynamic damper hasfour horizontal frames, it has the shape of a Japanese character “

”. So long as the dynamic damper is composed of the mass-adding part andthe viscoelastic part, needless to say, its shape is not limited to alattice.

Examples 1-7 of the high specific-gravity EPDM composition of thepresent invention and comparison examples 1-5 will be described indetail below.

The EPDM and other components such as additives were mixed with eachother at weight percentages shown in table 1. As will be describedbelow, different kinds of the EPDM were used in the examples and thecomparison examples. The mixing amount of a softener and that oftungsten will be described later. Unit in table 1 was parts by weight.

TABLE 1 Component Parts by weight EPDM 200 HAF (carbon) 40 Zinc oxide(two kinds) 5 Stearic acid 1 Age register IRGANOX 1010 0.5 IRGANOXMD1024 0.5 Powdery sulfur 1 Vulcanization accelerator M 1 Vulcanizationaccelerator TET 0.5 Vulcanization accelerator BZ 0.5 Vulcanizationaccelerator TTTE 0.5 In the table 1, IRGANOX 1010 meanspentaerythritol-tetrakis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate]. IRGANOX MD1024 means2′,3-bis[[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionyl]]propionohydraxyd.M means 2-melcapto.benzothiazole. TET means tetraethylthiuram disulfide.BZ means zinc dibutyl.dithiocarbamate. TTTE means telluriumdiethyldithiocarbamate.

As will be described later, oil was added to the components of each ofthe examples and the comparison examples as the softener by varying thekind and amount thereof. The components of each of the examples and the15 comparison examples were kneaded by a compact enclosed-type kneader(Mixlabo SW produced by Moriyama Co., Ltd.).

An amount of 400 g of tungsten powder (SG50-W produced by Tokyo TungstenCo., Ltd.) whose average diameter was 9 μm and specific gravity was 19.3was added to 25 g of the base material in a powdered state, namely,without surface-treating the tungsten powder with a coupling agent. Thenthe base material and the tungsten powder were kneaded by theenclosed-type kneader to obtain the high specific-gravity EPDMcomposition. The weight ratio of the tungsten powder to the highspecific-gravity EPDM composition was 94.1%.

Then the high specific-gravity EPDM composition was set in a die topress it at 170° C. for 15 minutes to obtain a sheet having a thicknessof 0.5 mm.

EXAMPLE 1

As the EPDM, Esprene 670F produced by Sumitomo Chemical Co., Ltd. wasused. The amount of diene was 4.0 wt %. The amount of ethylene was 66 wt%. The Mooney viscosity of the EPDM at 125° C. was 165. The Esprene 670Fcontained 100 wt % of oil for 100 wt % of the EPDM. The specific gravityof the high specific-gravity EPDM composition was nine.

EXAMPLE 2

As the EPDM, Esprene 512F produced by Sumitomo Chemical Co., Ltd. wasused. The amount of diene was 4.0 wt %. The amount of ethylene was 65 wt%. The Mooney viscosity of the EPDM at 125° C. was 66. As a softener, 30wt % of oil (Diana Process Oil PW380 produced by Idemitsu Kosan Co.,Ltd.) was added to 100 wt % of the EPDM. The specific gravity of thehigh specific-gravity EPDM composition was nine.

EXAMPLE 3

As the EPDM, Esprene 601F produced by Sumitomo Chemical Co., Ltd. wasused. The amount of diene was 3.5 wt %. The amount of ethylene was 59 wt%. The Mooney viscosity of the EPDM at 125° C. was 160. The Esprene 601Fcontained 70 wt % of oil for 100 wt % of the EPDM. The specific gravityof the high specific-gravity EPDM composition was nine.

EXAMPLE 4

As the EPDM, Esprene 673 produced by Sumitomo Chemical Co., Ltd. wasused. The amount of diene was 4.5 wt %. The amount of ethylene was 64 wt%. The Mooney viscosity of the EPDM at 125° C. was 110. As a softener,40 wt % of oil (Diana Process Oil PW380 produced by Idemitsu Kosan Co.,Ltd.) was added to 100 wt % of the EPDM. The specific gravity of thehigh specific-gravity EPDM composition was nine.

EXAMPLE 5

As the EPDM, Esprene 533 produced by Sumitomo Chemical Co., Ltd. wasused. The amount of diene was 4.5 wt %. The amount of ethylene was 58 wt%. The Mooney viscosity of the EPDM at 125° C. was 100. As a softener,40 wt % of oil (Diana Process Oil PW380 produced by Idemitsu Kosan Co.,Ltd.) was added to 100 wt % of the EPDM. The specific gravity of thehigh specific-gravity EPDM composition was nine.

EXAMPLE 6

As the EPDM, Esprene 512F was used similarly to the example 1. As asoftener, 50 wt % of oil (Diana Process Oil PW380 produced by IdemitsuKosan Co., Ltd.) was added to 100 wt % of the EPDM. The specific gravityof the high specific-gravity EPDM composition was nine.

Comparison Example 1

As the EPDM, Esprene 522 produced by Sumitomo Chemical Co., Ltd. wasused. The amount of diene was 5.0 wt %. The amount of ethylene was 56 wt%. The Mooney viscosity of the EPDM at 125° C. was 58. Oil was not addedto the EPDM. The specific gravity of the high specific-gravity EPDMcomposition was nine.

Comparison Example 2

As the EPDM, Esprene 582F produced by Sumitomo Chemical Co., Ltd. wasused. The amount of diene was 6.0 wt %. The amount of ethylene was 71 wt%. The Mooney viscosity of the EPDM at 125° C. was 67. As a softener, 30wt % of oil (Diana Process Oil PW380 produced by Idemitsu Kosan Co.,Ltd.) was added to 100 wt % of the EPDM. The specific gravity of thehigh specific-gravity EPDM composition was nine.

Comparison Example 3

As the EPDM, Esprene 524 produced by Sumitomo Chemical Co., Ltd. wasused. The amount of diene was 4.5 wt %. The amount of ethylene was 63 wt%. The Mooney viscosity of the EPDM at 125° C. was 25. Oil was not addedto the EPDM. The specific gravity of the high specific-gravity EPDMcomposition was nine.

Comparison Example 4

As the EPDM, Esprene 505A produced by Sumitomo Chemical Co., Ltd. wasused. The amount of diene was 9.5 wt %. The amount of ethylene was 50 wt%. The Mooney viscosity of the EPDM at 125° C. was 34. Oil was not addedto the EPDM. The specific gravity of the high specific-gravity EPDMcomposition was nine.

EXAMPLE 7

As the EPDM, Esprene 670F produced by Sumitomo Chemical Co., Ltd. wasused. The amount of diene was 4 wt %. The amount of ethylene was 66 wt%. The Mooney viscosity of the EPDM at 125° C. was 165. As a softener,170 wt % of oil (Diana Process Oil PW380 produced by Idemitsu Kosan Co.,Ltd.) was added to 100 wt % of the EPDM. The specific gravity of thehigh specific-gravity EPDM composition was nine.

Comparison Example 5

As tungsten-containing chloroprene commercially available, a heavy metalsheet (HMS-09C produced by Sumitomo Electric Industries, Ltd.) was used.

The hardness (standard A) of the sheet (sheet could not be formed in thecomparison example 4) of the examples 1-7 and the comparison examples1-5 was measured. The tensile strength and tensile elongation of eachsheet were also measured in the tensile test. A weatherability test wasalso conducted to evaluate whether blooming occurred on the sheets. Thetest method and the evaluation method will be described later. Table 2shown below indicates the kind of the EPDM, the kind of the EPDM,components thereof, and results of evaluations.

TABLE 2 E1 E2 E3 E4 E5 E6 EPDM (Esprene 670F 512F 601F 673 553 670Fnumber) Diene amount (wt %) 4 4 3.5 4.5 4.5 4 Ethylene 66 65 59 64 58 66amount (wt %) Mooney 165 66 160 110 100 165 viscosity (125° C.) Oilamount (wt %) 100 30 70 80 40 150 Hardness (A) 72 85 84 85 80 54 Tensile5.1 9.7 8.4 9.4 9.59 3.84 strength (MPa) Tensile 590 195 681 170 190 609elongation (%) Blooming ◯ ◯ ◯ ◯ ◯ ◯ CE1 CE2 CE3 CE4 E7 CE5 EPDM (Esprene522 582F 524 505A 670F — number) Diene amount (wt %) 5 6 4.5 9.5 4 —Ethylene 56 71 63 50 66 — amount (wt %) Mooney 58 67 25 34 165 —viscosity (125° C.) Oil amount (wt %) 0 30 0 0 170 — Hardness (A) 70 8877 — 39 88 Tensile 2.6 10.6 2.8 — 2.7 4.3 strength (MPa) Tensile 350 200320 — 605 530 elongation (%) Blooming X X ◯ X Δ Δ where E denotesexample, and CE denotes comparison example.Measurement of Hardness

The surface hardness of each vulcanized high specific-gravity EPDMcomposition was measured by the method specified by JIS K-6253 (testerdurometer type A).

Tensile Test

In JIS 3 dumbbell shape, a tensile test was conducted on the specimensheets at a tensile speed of 500 mm/min to measure the strength andelongation thereof at the time of breakage thereof.

Weatherability Test

A weatherability test was conducted by a sunshine super-longweatherometer (WEL-SUN-HC.B type produced by Suga tester Co., Ltd.) for120 hours to check whether each specimen sheet had blooming or not.

In the examples 1-6, the EPDM whose diene amount and ethylene amountwere in the specified range was used, and the oil was used in thespecified range. Therefore the surface hardness of the vulcanizedspecimen sheet was less than 90, and the tensile strength thereof wasnot less than 3 MPa. Thus the specimen sheet did not bloom and had goodweatherability. It was confirmed that the specimen sheet of the examples1-6 was optimum as the material for the mass-adding part of the dynamicdamper.

In the example 7, the same EPDM as that of the examples 1 and 6 wasused. However since a comparatively large amount of oil serving as thesoftener was used, the specimen sheet had a lower strength and had moreblooming than the examples 1-6, but the extent of the blooming has noproblem in the use thereof. Thus it was confirmed the specimen sheet ofthe example 7 was also suitable as the material for the mass-adding partof the dynamic damper.

In the comparison example 1, since the amount of ethylene was less than58 wt %, the strength of the specimen sheet was low. Since the amount ofdiene was more than 4.5 wt %, the specimen sheet had blooming and lowweatherability.

In the comparison example 2, although the specimen sheet had a highstrength, the diene amount was more than that of the comparisonexample 1. Thus the specimen sheet bloomed more than the specimen sheetof the comparison example 1 and had low weatherability.

Since the EPDM of the comparison example 3 had a low Mooney viscosity,the specimen sheet had a low strength.

Since the EPDM of the comparison example 4 had a small amount ofethylene and a low Mooney viscosity, the moldability of the specimensheet was so low that the specimen sheet was broken into pieces andcould not be processed into a sheet. In the weatherability test of abroken piece, it had blooming and had a poor weatherability.

The specimen sheet of the comparison example 5 had no problem in itshardness and strength. However, in the weatherability test, the surfacethereof deteriorated and had blooming and inferior weatherability.

From the above, it can be confirmed that by using the EPDM having anoptimum condition, adding an optimum amount of softener to the EPDM, andusing a specified amount of tungsten powder, it is possible to form thehigh specific-gravity material that was appropriately soft as thematerial for the mass-adding part of the dynamic damper, can beprocessed in combination with other materials, had no problem instrength, and did not have blooming.

The example 8 in which the dynamic damper is composed of the highspecific-gravity EPDM composition of the present invention and thecomparison example 6 will be described in detail below.

EXAMPLE 8

As the material for the mass-adding part, the sheet prepared in theexample 1 was used.

As the material for the viscoelastic part, the material composed of thecomponents shown in table 3 was used. The components were kneaded withan enclosed-type kneader.

TABLE 3 Component Parts by weight Esprene 532 (EPDM) (produced bySumitomo 100 Chemical Co., Ltd.) Diana process oil Px-90 (produced byIdemitsu 250 Kosan Co., Ltd.) Zinc oxide (two kinds) 150 Stearic acid 5Powdery sulfur 1 Vulcanization accelerator M 1.0 Vulcanizationaccelerator TET 0.5 Vulcanization accelerator BZ 0.5 Vulcanizationaccelerator TTTE 0.5 Titanium oxide 10 Where M, TET, BZ, and TTTE denotethe same substance as that shown in table 1 respectively.

The mass-adding part and the viscoelastic part are layered on each otherand set in a die. Thereafter the laminate was pressed at 170° C. for 20minutes and vulcanized into the shape of a dynamic damper. The shape ofthe dynamic damper was similar to that of the example 1.

Comparison Example 6

As the mass-adding part, a heavy metal sheet (produced by SumitomoElectric Industries, Ltd.) having a thickness of 0.6 mm was used. Theheavy metal sheet was made of tungsten-containing chloroprene rubber.The viscoelastic part was similar to that of the example 8. A dynamicdamper having the same configuration as that of the example 7 wasprepared by a method similar to the above-described method.

The dynamic damper of the example 8 and that of the comparison example 6were installed at three and nine o'clock positions of the head part ofthe tennis racket respectively. The out-of-plane secondary naturalfrequency, damping ratio, in-plane tertiary natural frequency, anddamping ratio of the dynamic damper-mounted tennis racket of each of theexample 8 and the comparison example 6 were measured. The weatherabilityof each dynamic damper was evaluated by conducting an outdoor exposuretest. The test method and the measuring method will be described later.Table 4 shows results of the evaluation.

TABLE 4 Out-of-plane Damp- In-plane Damp- secondary ing tertiary ingOutdoor natural ratio natural ratio exposure frequency (Hz) (%)frequency (Hz) (%) test E8 422 4.5 371 5.5 ◯ CE6 421 4.4 370 5.3 ΔBlooming occurred where E denotes example, and CE denotes comparisonexample.Measurement of Natural Frequency and Damping Ratio

The method of measuring the natural frequency of each of the tennisrackets TR and the damping ratios thereof is shown in FIGS. 5 and 6. Tomeasure them with high accuracy, an acceleration pick-up meter 73 wasmounted on a maximum amplitude position of the tennis racket TR in eachvibration mode. In this state, the maximum amplitude position of thetennis racket TR was hit with an impact hammer 71 to impart vibrationsto the tennis racket TR. No gut was stretched on the gut-stretched partof the racket frame. As shown in FIGS. 7 and 8, the natural frequency ofthe tennis racket TR and its damping ratio were measured by a freesupporting method of hanging the tennis racket TR with a string. Aninput vibration (F) measured with a force pick-up meter installed on theimpact hammer 71 and a response vibration (a) measured with theacceleration pick-up meter 73 were inputted to a frequency analyzer 74(manufactured by Furet Packard Corp., dynamic single analyzer HP 3562A)through amplifiers 72 and 70 to analyze the input vibration (F) and theresponse vibration (α). This method was carried out by supposing thatthe rigidity of the racket frame was linear. Table 4 shows the resultsof measurement on tennis racket of each of the examples and comparisonexamples.

A transmission function, in a frequency region, obtained by the analysiswas determined to obtain the out-of-plane secondary frequency and thein-plane tertiary natural frequency of the racket frame. Thevibration-damping ratio (ζ) was computed with reference to FIG. 6 byusing the following equation:ζ=(½)×(Δω/ωn)To=Tn/√2Measurement of Out-of-plane Secondary Natural Frequency

As shown in FIG. 7, the out-of-plane secondary natural frequency is asecond peak which appears with respect to a low frequency when thetennis racket 1 set in a free supporting state of hanging the tennisracket 1 with a string is hit on its back. More specifically, theout-of-plane secondary frequency is a frequency at the time when thetennis racket 1 (before deformation) shown in FIG. 9A vibrates in theout-of-plane secondary mode, as shown in FIG. 9B (side view of thetennis racket).

Measurement of In-plane Tertiary Natural Frequency

As shown in FIG. 8, the in-plane tertiary natural frequency is a thirdpeak which appears with respect to the low frequency when the tennisracket 1 set in a free supporting state of hanging the tennis racket 1with a string is hit from the outside. More specifically, the in-planetertiary natural frequency is a frequency (before deformation), shown inFIG. 10A, at the time when the tennis racket 1 vibrates (deforms) in thein-plane tertiary mode, as shown in Fig. 10B.

Outdoor Exposure Test

An outdoor exposure test was conducted on the tennis racket on which thedynamic damper of the example 8 was mounted and the tennis racket onwhich the dynamic damper of the comparison example 6 was mounted. In theoutdoor exposure test, the tennis rackets were exposed to rain, wind,and sunshine for two months of August and September. Table 4 shows theresult of evaluation.

As described above, the mass-adding part of the dynamic damper of theexample 8 was composed of the high specific-gravity EPDM composition,and the mass-adding part of the dynamic damper of the comparison example6 was composed of the tungsten-containing chloroprene rubbercommercially available. The vibration-damping performance of the dynamicdamper of the example 8 was equal to that of the dynamic damper of thecomparison example 6. As a result of exposure to the scorching heat ofthe sun in two months in summer, the dynamic damper of the example 8 didnot have blooming and was superior in weatherability, whereas thedynamic damper of the comparison example 6 deteriorated a little on itssurface. It can be confirmed that the dynamic damper composed of thehigh specific-gravity EPDM composition of the present invention issuperior in weatherability and in vibration-damping performance.

Evaluation was made on the performance of the radioactive ray shieldingmaterial composed of the high specific-gravity EPDM composition of eachof the examples 1-7. To do so, using the high specific-gravity EPDMcompositions (specific gravity: nine) of the examples 1-7, sheets havinga thickness of 1 mm were prepared.

As a comparison example 7, a lead plate in the same configuration asthat of the examples 1-7 was used. As a comparison example 8, alead-containing sheet (specific gravity: four) having the sameconfiguration as that of the examples 1-7 was used.

Measurement of Radioactive Ray Shielding Performance

The radioactive ray absorption characteristics of the sheets composed ofthe high specific-gravity EPDM composition of the examples 1-7respectively, the lead plate of the comparison example 7, and thelead-containing sheet of the comparison example 8 were measured byirradiating them with X-rays of 6 MeV.

The result of the measurement was that the radioactive ray shieldingperformance of the sheet of each of the examples 1-7 was 95% of that ofthe lead plate of the comparison example 7 and 1.9 times as large asthat of the lead-containing sheet of the comparison example 8. It can beconfirmed that the radioactive ray shielding performance of the sheet ofeach of the examples 1-7 is almost equal to or higher than that of theconventional lead plate or the conventional lead-containing sheet.

As apparent from the foregoing description, the present inventionprovides the high specific-gravity EPDM composition which is the mixtureof the EPDM containing diene and ethylene at a wt % in the specifiedrange and having a Mooney viscosity in the specified range at 125° C.and the powdery material containing powder whose specific gravity is notless than 12 as a main component thereof. The powdery material is addedto the EPDM at not less than 80 wt % nor more than 97.5 wt % of thewhole amount (total weight of EPDM, additives, and powdery material) ofthe high specific-gravity EPDM composition. Thus the highspecific-gravity EPDM composition does not have blooming and hasimproved weatherability. Further the high specific-gravity EPDMcomposition is high in moldability and processability.

Further the high specific-gravity EPDM composition is soft, has a highspecific gravity and strength, and does not pollute environment.Therefore the high specific-gravity EPDM composition can be used for thedynamic damper, the radioactive ray shielding material, thevibration-damping/sound insulation sheet, the soundproof material, andthe like.

Further the high specific-gravity EPDM composition is appropriatelysoft, can be processed together with other materials, and does not haveany problems in strength nor blooming. Therefore the highspecific-gravity EPDM composition can be preferably used as the materialfor the mass-adding part that is layered on the viscoelastic part.Moreover since the high specific-gravity EPDM composition has a highspecific gravity, it is possible to make the volume and thickness of thedynamic damper small. Thus the dynamic damper does not disturb a playerduring the use of a tennis racket and is unnoticeable in appearance.Further since the dynamic damper is soft, there is no fear that theplayer is injured thereby, even when the player touches the dynamicdamper or strikes it against others by mistake.

Further since it is possible to form the dynamic damper as a thin sheet,a player can play tennis without caring about the dynamic damper.Furthermore since a small air resistance acts on the dynamic damper, theplayer has high operability in using the tennis racket. Although thedynamic damper is smaller than the conventional one, the former providessufficient vibration-damping performance.

The radioactive ray shielding material of the present invention does notpollute environment and has high moldability, processability, heatresistance, and strength, and radioactive ray shielding performance.Thus the radioactive ray shielding material can be easily processed intovarious configurations and preferably used as a replacement of metal andfor radiation therapy, atomic power plants, and industrial and medicalradioactive ray inspection machines.

1. A high specific-gravity EPDM composition comprising: a mixture ofEPDM containing diene at less than 4.5 wt % and ethylene at not lessthan 58 wt % nor more than 80 wt % and having a Mooney viscosity ML₁₊₄not less than 50 nor more than 170 at 125° C. and a powdery material,containing powder whose specific gravity is not less than 12 as a maincomponent thereof, added to said EPDM at not less than 80 wt % nor morethan 97.5 wt % of a whole amount of said high specific-gravity EPDMcomposition.
 2. The high specific-gravity EPDM composition according toclaim 1, wherein less than 150 wt % of a softener is added to 100 wt %of said EPDM.
 3. The high specific-gravity EPDM composition according toclaim 1, wherein said powdery material is tungsten, a tungsten compoundor a tungsten based alloy.
 4. The high specific-gravity EPDM compositionaccording to claim 1, having a specific gravity not less than 4.5 normore than 13.1.
 5. The high specific-gravity EPDM composition accordingto claim 1, wherein a surface hardness of said vulcanized highspecific-gravity EPDM composition measured by a method specified by JISK-6253 (tester type A) is less than 90; and a tensile strength of saidvulcanized high specific-gravity EPDM composition is not less than 3MPa.
 6. A dynamic damper composed of a viscoelastic part and amass-adding part, wherein a high specific-gravity EPDM compositionaccording to claim 1 is used as said mass-adding part.
 7. A tennisracket installing a dynamic damper according to claim 6 on at least oneportion of a head part surrounding a ball-hitting face of a racket frameor/and at least one portion of a throat part thereof.
 8. The tennisracket according to claim 7, wherein a mass-adding part is composed ofsaid high specific-gravity EPDM composition molded in a shape of a sheetand layered on said viscoelastic part and integrated therewith.
 9. Thetennis racket according to claim 8, wherein a thickness of saidsheet-shaped mass-adding part is set to not less than 0.3 mm nor morethan 1.7 mm.
 10. A radiation-shielding material comprising a highspecific-gravity EPDM composition according to claim 1.